Precise measurements of the 18O to 16O ratio of the seawater samples will be made using a dual-inlet isotope ratio mass spectrometer following well-accepted methods. To make these measurements, CO2 gas will be isotopically equilibrated with the sample under controlled conditions in the presence of a catalyst and then transferred to the mass spectrometer for analysis. Each sample will be analyzed twice, providing a verification of the analysis and yielding a high precision final result.